dyno, the folks who have already replied have some good info.
The standard combat cartridge for the M14 family is the 147 grain FMJ flat based M80 Ball cartridge.
As stated, the M118 Special Ball is a 173 grain FMJ BT designed for better long range accuracy.
The M118 LR (long rage) Ball is a 173 grain HPBT.
The The M852 Match round is a 168 grain BPBT (essentially a Sierra MatchKing).
So, your rifle should be happy with most any spitzer bullet in the 150-175 grain range. Some round or flat nosed bullets may have feeding issues. BTW, the HPBT rounds have been used in combat for years. Since the HP is for accuracy purposes and not to increase the severity of wounds they are perfectly legal for combat.
As you have already seen you will want to stay to powders that burn like the military powders so that the rifle get an appropriate amount of energy to function as designed. Due to gas system fouling issues with dirty powder you'll want to stay with clean burning powders. Hodgdon and IMR 4064 and 4895 are great choices. Standard or match large rifle primers both work fine.
M14 family rifles can be hard on brass so take some extra time in inspecting your brass before reloading. The most common issues tend to be a thinning of the side walls just above the base web and neck splits. Many rifles will leave a ding in the sidewall where the case hits the side of the receiver and pivots out of the action and it is not an issue so don't worry about it.
The neck splits are easy to see and usually don't crop up for several reloads. The cracks above the base web can be felt in the inside of the case before you'll see them in the outside of the case. If you take a hunk of wire such as coat hanger wire, flatten the end and then bend the very tip so that the flat is perpendicular to the long axis of the wire you can use it feel a crack starting inside the case. The first time you find one you'll know it. Those cracks usually show up at around 4-5 reloads and are somewhat dependent on how much you re-size the cases and how hot the loads are.
Some folks try to cut down on working the brass by doing only minimal sizing but long experience with these rifles indicates that full length sizing the brass to factory specs is the best way to insure flawless functioning.
You don't state whether the flyers you are getting are in a consistent place or not but there are a few things to check with these rifles that you can take to the bank.
Crud or fouling in the gas system will exhibit itself as vertical stringing since it can effect the muzzle velocity and the stringing can be either consistent or occasional. They can also lead to cycling failures in extracting, ejecting, feeding, chambering and locking. Crud slows the action down and dry carbon build up changes the volume of gas available to work the action.
If you lock the bolt to the rear and tilt the rifle up and down you should hear the gas piston sliding back and forth and it should make a soft click when it hits the ends of the cylinder. If not then you need to clean the system and make sure that everything is dry (and stays that way) when you reassemble it. Ream the carbon out of the piston and plug but don't use anything even a bit abrasive on the outside of the piston or in the bore of the cylinder. The plug has a step in it's interior diameter so be sure to get all the way down to the bottom of the smaller bore. The outside of the piston tends to be self cleaning but any carbon build up in the piston and plug changes the volume of the gas system. A lot of carbon increases the velocity since it doesn't bleed off the correct amount of gas and the resulting higher pressure means that the operating rod and bolt both get pounded a lot harder than they need to. If you want me to I'll dig out the drill bits I use for this and post the sizes. I epoxied them into handles and ground off the tips to make flat ended carbon reamers out of them
Always clean the rifle upside down (with the sights down, the magazine well up) and the muzzle lightly downhill so that solvents and crud don't get in the action or gas system. Use a bore guide (a cut down 12 gauge shotgun shell with a hole drilled through the primer pocket works as well as the factory ones) and put patches on inside the breech and then pull them out the muzzle. That will prevent you from getting solvents or crud pushed into the action.
While you have the action out of the stock look at the inside bottom of the front barrel band and the bottom front metal of the stock. If the area where they meet is not glass slick use a very fine abrasive cloth or paper to polish them. In any case put a dab of grease there when you reassemble the rifle. If those surfaces are not dead smooth and lubricated than when the rifle moves and flexes under recoil and in handling the stock and barrel will not return to the same place each time you squeeze a shot off. For the accuracy you seek it may be all that is needed to correct your flyers.
Assuming you are hunting deer sized game the 150 and 165 Grain Sierra GameKing bullets are the closest thing going to the match style bullets in your rifle but any 150-175 grain bullet should work just fine as long as the nose shape does not cause feeding issues. 180 grain bullets are pushing the limit of the action so I'd back off the powder maybe two or three grains to be sure the bolt isn't getting smacked into the rear of the receiver. I've seen a few rifles with the receiver cracked right behind the rear sight by loads a bit too hot.
I know this was more than you asked for and you may already have had all this info but with M14 style rifles it all kind of goes together sometimes.
Lance
